The 3GPP Release 12 specifications define Dual Connectivity (DC) (see, for example, Non Patent Literature 1 and 2). DC is similar to but different from Coordinated Multipoint (CoMP) introduced in the 3GPP Release 11 specifications. In particular, in DC, different carrier frequencies are used for a macro cell and a small cell (picocell), a radio terminal (i.e., User Equipment (UE)) executes two Medium Access Control (MAC) entities (that is, the UE uses radio resources provided by two independent schedulers, one of which is in a master base station (i.e., Master eNodeB (MeNB)) and the other is in a secondary base station (i.e., Secondary eNB (SeNB))), and the UE needs to support simultaneous use of at least two UL carriers.
In DC of 3GPP Release 12, the UE is connected to one MeNB and one SeNB. The UE configured with DC (DC UE) uses two MAC entities and at least two Radio Link Control (RLC) entities for respective data flows on the MeNB and the SeNB. The MeNB in DC is an eNB that terminates S1-MME for the DC UE. The SeNB in DC provides additional radio resources for the DC UE. One or more serving cells provided for the UE by the MeNB are referred to as a Master Cell Group (MCG), and one or more serving cells provided for the UE by the SeNB are referred to as a Secondary Cell Group (SCG). The MCG is one or more serving cells associated with the MeNB and includes a Primary Cell (PCell) and optionally one or more Secondary Cells (SCells). The SCG is composed of one or more serving cells associated with the SeNB and includes a Primary Secondary Cell (PSCell) and optionally one or more Secondary Cells (SCells). In DC of 3GPP Release 12, the PSCell is a special cell in the SCG and is different from a normal SCell in that the PSCell is configured with the uplink (i.e., PUCCH) for the UE.
In the control plane architecture for DC, there is only one S1-MME per DC UE. The S1-MME is defined between the MeNB and a Mobility Management Entity (MME), and the MeNB terminates the S1-MME. The signaling between the MeNB and the SeNB regarding the DC UE is performed using a signaling interface (i.e., X2-C) between the eNBs.
For DC, two user plane architectures are allowed. In the one architecture, S1-U is only terminated in the MeNB and the MeNB forwards user plane data to the SeNB using X2-U. In the other architecture, S1-U can also be terminated in the SeNB.
Accordingly, three types of radio bearers, i.e., MCG bearer, split bearer, and SCG bearer, are defined for DC. Regarding the MCG bearer, the MeNB terminates an S1-U connection with a Serving Gateway (S-GW), and Radio protocols of the MCG bearer are only located in the MeNB to use MeNB resources. Thus, the SeNB is not involved in transferring user plane data of the MCG bearer on the Uu interface.
Regarding the split bearer, the MeNB terminates an S1-U connection with an S-GW, and Packet Data Convergence Protocol (PDCP) data (i.e., PDCP Protocol Data Unit (PDCP PDU)) is transferred between the MeNB and the SeNB. Thus, both the MeNB and the SeNB are involved in transferring user plane data of the split bearer on the Uu interface.
Regarding the SeNB bearer, the SeNB terminates an S1-U connection with an S-GW, and Radio protocols of the SeNB bearer are only located in the SeNB to use SeNB resources. Thus, the MeNB is not involved in transferring user plane data of the SCG bearer on the Uu interface.
Some overall procedures regarding DC are defined in Non Patent Literature 1 (see, for example, Section 10.1.2.8 of Non Patent Literature 1).